A monolithic microwave integrated circuit (MMIC) is an integrated circuit which operates at microwave frequencies. Typically, a MMIC is designed to operate in a band of frequencies centered on a particular operating frequency. A MMIC may have input and output connections for signals at or near the operating frequency, and, in addition, a MMIC may have low-frequency or direct current (DC) connections, known as bias connections. Bias connections may provide power or control signals to the components in the MMIC, or they may provide for the output of low-frequency signals from the MMIC. MMIC amplifiers, for example, require DC power, MMIC modulators require modulation signals, and MMIC detectors may produce low-frequency output signals.
It is generally preferred that the microwave signals at or near the operating frequency not be able to propagate into or out of the MMIC through the bias connections. Otherwise, a microwave signal may propagate out of the MMIC through a bias connection, reflect from components outside of the MMIC, and propagate back into the MMIC, again through a bias connection; such unwanted interactions with circuitry outside the MMIC may lead to ripple in its frequency response, or to oscillations. Because the MMIC designer has limited control over external circuitry that may later be connected to the MMIC, it is desirable to make the operation of the MMIC as nearly as possible independent of such external circuitry. In a MMIC amplifier, efficiency may be a further reason for preventing transmission of microwave signals through bias connections: any net microwave power loss at a bias connection corresponds to power that can not be made available at the circuit's microwave output connection.
Microwaves may be prevented from propagating into or out of the MMIC by providing a microwave short circuit to ground at some point on the bias conductor. Such a short circuit will reflect microwave signals. A sufficiently large capacitor, for example, connected between a bias conductor and ground will approximate a short circuit to ground for microwave signals. Provided the capacitor is not too large, it will also approximate an open circuit at low frequencies, allowing the bias signals to propagate freely into or out of the MMIC.
A large capacitor, however, may occupy a large area in the MMIC. In MMIC design, the physical size of the integrated circuit, or “chip,” is often constrained, and a MMIC designer must try to fit the circuit being designed into a small area. The impedance at microwave frequencies of a capacitor is inversely proportional to its area. As a result, a capacitor which occupies a small area on the chip may not provide a sufficiently low impedance at microwave frequencies.
Other characteristics may be desirable in a bypass circuit. Unless proper design precautions are taken, a MMIC amplifier may oscillate at frequencies other than the operating frequency. This may be prevented by providing loss in the microwave signal path, for example by installing a series resistor in the bias conductor. Such a resistor, however, will add loss at all frequencies, including the operating frequency and DC, wasting both DC supply power and microwave power at the operating frequency. It is desirable, therefore, that the bias circuit provide loss primarily at frequencies at which the MMIC amplifier might otherwise oscillate.
Thus, there is a need for a bypass circuit which provides a low impedance at a microwave operating frequency, while consuming a smaller amount of chip area than a capacitor with a similar impedance at the same frequency. Further, there is a need for a bypass circuit capable of providing significant loss at microwave frequencies different from the operating frequency.